The present invention relates to conditioning processes for preparing polymer surfaces for subsequent metal plating through electroless deposition processes. More particularly, this invention relates to such processes as applied to extremely thin substrates, less than about 1 mil in thickness, particularly for polyimide and polyester films.
This invention has particular utility in the field of electroless plating. Generally speaking, electroless plating requires a so-called activation or catalyzation step during which a substrate surface, to be electrolessly plated with a metal, has placed thereon a material, usually a metal salt. This metal salt is capable of reducing the plated metal from an electroless bath without the use of an electric current. Catalyzation by such a material (called a "catalyst" or an "activator") is referred to as such because the materials used, usually the salts of the precious metals (palladium, platinum, gold, silver, etc.) serve as reduction catalysts in an autocatalytic electroless plating process. Given a suitable surface for electroless deposition, an experienced practitioner can produce a smooth metal layer upon the substrate having good adhesion with little if any difficulty using widely available standard processing solutions.
Even though significant progress has been made in the art of plating metals on plastics, the adhesion between the coating and the plastic still leaves much to be desired. Poor adhesion between the plastic and the metal plated thereon allows differential dimensional changes with temperature and stress caused by a sub-optimum composition of the plating bath solution, either of which may result in warping, blistering, and cracking of the metallized product. Consequently, strong adhesion between a plastic substrate and the plated metallic layer is essential for any application in which the product is subjected to significant temperature fluctuations and subsequent mechanical processing.
Plastics show a relatively poor affinity for metal, and, to promote a stronger bond between a plastic substrate and a metallic coating, the prior art frequently resorted to roughening the plastic surface to provide locking or keying between the surface and its coating. The surface of the plastic substrate is normally relatively glossy and quite hydrophobic. Consequently, this surface is unreceptive to aqueous solutions used in electroless metal deposition. Since the sensitizing and activating solutions used for electroless plating will not wet the surface, the metal ions are not adsorbed onto the surface and deposition of the metal cannot proceed.
Roughening the surface of the hydrophilic substrate by mechanical means has been common practice in plating plastic materials. The first techniques used to this end were accomplished by some sort of mechanical deglazing, such as scrubbing with an abrasive slurry, dry rolling or abrasive (sand) blasting. This left a rough surface with significant topographic deviations from the prior planar surface, in turn requiring a relatively thick metal plating layer to return the object to a smooth surface condition. Clearly, this technique could not be used for thin substrates.
Roughening can also be accomplished by chemical means. Caustic and acidic etch solutions and solvents have been employed, depending upon the polymer composition. These acidic etch solutions commonly used sulfuric acid and chromate salts, mainly for polyesters but not polyimides. It should also be noted that acid-chromate etch solutions are not desirable from the environmental standpoint, due to the expensive requirement of removing the chrome and chromate ions from the spent solutions.
It was also discovered that preceding the acidic etch step with a caustic bath would improve the adhesion of the metal in subsequent steps. However, the etching removes a portion of the substrate surface and can result in an uneven surface, and its use is suspect when a thin substrate film is to be used. The acid pH is associated with the Pd.sup.++ bath which uses HCl to solubilize the Pd metal. Also an acid rinse may be used after the caustic etch to quench the reaction.
Weight is always a concern for anyone developing equipment which is to be used as a component on aircraft, spacecraft or portable equipment such as laptop computers and cellular telephones. The plastic substrates used for printed circuits in such components are one area in which further weight reductions could be realized if there were a process available that could condition these thin substrates (less than 1 mil in thickness-0.001 inch) successfully. Heretofore no such process has been known to exist. Present conditioning techniques are limited in their application to substrates greater than 1 mil in thickness due to the harshness of the chemical solutions and the high reaction rates caused by high processing temperatures. Although these potential substrates are offered by their various manufacturers at extremely thin thickness', i.e. Mylar.RTM. (a polyethylene terephthalate-PET) is available in thickness' down to 0.00006 inch and Kapton.RTM. (a polyimide) is available down to 0.0003 inch, there is no known method of effectively conditioning these very thin substrates for subsequent electroless metal deposition.
Suppliers of these films suggest the use of shorter exposure for thinner substrates in caustic baths operating at higher temperatures, i.e. 5-10 minutes in a 50-70.degree. C. bath. However, the polycrystalline nature of these thin films (30-35% crystalline/65-70% amorphous) leads to a non-uniform activation and subsequent imperfect metallization. It is postulated that the amorphous portions of the polymer are more aggressively attacked by the caustic.
U.S. Pat. No. 5,441,770 teaches the use of an aqueous solution of 25-60 wt % inorganic hydroxide (NaOH or KOH)+a wetting agent (about 1% of the total)+organic hydroxide (glycol or alcohol at less than 20% of the total), followed by use of an etchant solution of potassium or sodium permanganate (0.1-0.5 Molar), followed by neutralization by a hydrogen peroxide solution for conditioning of a polyimide substrate. These steps are conducted at 75-90.degree. C. This process is much too harsh for use on the very thin substrates contemplated herein.
U.S. Pat. No. 5,015,517 discloses the use of a conditioning/etching mixture of KOH (15-40 wt %), a diamine (3-15 wt %) and alcohol (10-50 wt %) to surface treat a polyimide substrate. U.S. Pat. No. 4,820,553 discloses the treatment of PET and polyamides with an aqueous mixture of a source of hydroxyl ions (typically KOH at 20 wt %) and a water soluble organic solvent (preferably methanol at 33-50 wt % of the solution). Here again the high concentrations of the hydroxyl ions render these solutions unsuitable for use on the very thin substrates contemplated herein.
Based on the above and foregoing, it can be appreciated that there presently exists a need in the art for a method capable of providing a properly conditioned very thin polymer substrate for subsequent electroless metal deposition, which overcomes the above-described drawbacks and shortcomings of the presently available technology. The present invention fulfills these needs in the art.